1. Field of the Invention
The present invention relates to an exhaust gas treatment system and an exhaust gas treatment method for removing pollutants such as SO3 in combustion exhaust gas.
2. Description of Related Art
The combustion exhaust gas produced at the time of combustion of fossil fuel or wastes in a combustion furnace, contains pollutants such as sulfur trioxide (SO3), hydrogen fluoride (HF), hydrogen chloride (HCl), mercury chloride (HgCl), hydrogen sulfide (H2S), carbonyl sulfide (COS) and so forth. Since these pollutants have a damaging effect on the environment when discharged into the atmosphere, they are required by laws and regulations to be treated to be below a predetermined concentration. Up until now, various kinds of exhaust gas treatment system for achieving this have been proposed.
SO3 is described as an example of the pollutants.
SO3 is produced primarily when some portion of SO2, which has been produced in combustion of fuel containing sulfur (heavy oil, coal and so forth), is oxidized under a high temperature environment. Therefore, the SO3 content ratio to SO2 is a few percent. However, since SO3 causes blockage and corrosion of air heaters, and corrosion of flues, and causes bluish smoke when cooled and discharged from a stack, it is preferable to suppress its discharge concentration to a few ppm or less.
As a method of removing SO3, the ammonia gas injection method, in which ammonia gas is injected into combustion exhaust gas, is commonly known.
In the ammonia gas injection method, ammonium sulfate and dust are produced as products from ammonia gas and the combustion exhaust gas. The ammonium sulfate is generally collected as a solid state substance by a dry electrostatic precipitator. However, since heavy metals contained in fuels such as heavy oil are also collected with the ammonium sulfate, it needs to be separately treated before it is discharged into the environment. Accordingly, the ammonia injection method has a disadvantage of facility cost and operation cost.
Moreover, since a large amount of ammonia gas is injected in order to produce ammonium sulfate, ammonia is occasionally incorporated into desulfurization effluent of a wet desulfurization system provided on the downstream side. In this case, desulfurization effluent treatment is required to meet environmental standards.
Moreover, since ammonia gas needs to be constantly supplied, a large consumption of ammonia gas is an obstacle to operation cost reduction.
On the other hand, in some cases, an wet type electrostatic precipitator or a dielectric flue gas treatment system (Japanese Patent No. 3564366) is installed on the downstream side of a flue in order to remove dust produced by ammonia injection, and ash in the combustion exhaust gas.
A wet electrostatic precipitator and a dielectric flue gas treatment system also remove SO3 while removing dust. However, even when using these, in the case of highly concentrated SO3, the use of a wet electrostatic precipitator or a dielectric flue gas treatment system having an economically feasible treatment capacity may not achieve a sufficient level of SO3 removal.